1. Field of the Invention
This invention relates to an optical recording medium and, more particularly, to an optical recording medium having plural recording layers.
2. Description of the Related Art
Among read-only optical discs, there is such a disc in which a reflective metal layer, composed mainly of aluminum, is formed as a recording layer on a transparent substrate on which are formed crests and valleys corresponding to the data information. An example of such disc is a so-called compact disc. In such optical disc, attempts are made for increasing the surface recording density for increasing the recording capacity.
On the other hand, there is recently developed an optical disc in which the recording layer is formed by multiple layers for increasing the recording capacity. An example of this type of disc is a digital video disc (DVD). For such digital video disc, there is proposed a disc of the type in which two recording layers are formed on the same substrate such that the effective recording capacity is twice that of the single-layer disc.
If, in the optical disc having plural recording layers, the laser light is illuminated on one of the major surfaces of the disc for independently reproducing information signals of the respective recording layers, it is required of the recording layer on which the laser light is incident that such recording layer be of higher transmittance for transmitting the laser light to the other recording layer lying further than the light incident side layer. Thus, in such multi-layer optical disc, a metal reflective layer thinner in thickness than the layer used for a single-layer optical disc, or a dielectric layer having a higher refractive index and lower light absorption, is used as a laser light incident side recording layer, so that more than one-half of the incident light intensity is transmitted through this recording layer. This enables the laser light to be accorded to the second and even third or fourth layers, while also enabling the return light to be received from these recording layers. The recording layers of the multi-layer optical disc are termed first, second, third, . . . n'th recording layers, looking from the reproducing laser light incident side.
Meanwhile, a dye material, a phase transition material or a magneto optical recording material is used as a recordable type recording material for the optical disc. If these recordable type recording materials can be used for the multi-layer type optical disc, the optical disc is expected to be improved further in utility. Such optical disc may possibly be configured by layering plural recordable type recording layers or by using the combination of the read-only recording layers and the recordable type recording layers.
However, with the recordable type recording layers, it is difficult to raise the transmittance for the following reason, such that it is difficult to realize the configuration in which the recordable type recording layer is used as the first recording layer.
That is, the magneto optical recording material or the phase transition material is a heat mode type recording material in which the temperature rises by absorption of the laser light and in which pits are formed by such rise in temperature. Thus, for effecting the recording, the light needs to be absorbed to a more or less extent. For this treason, the materials need to be lowered in transmittance.
Moreover, for obtaining a high playback signals level, or optimizing heat response characteristics during recording, the usual practice is to have the recording layer sandwiched between two dielectric layers and to form a reflective metal layer composed mainly of aluminum or gold thereon.
If it is attempted to reduce the overall thickness of a three-layer recording portion, comprised of the recording layer sandwiched between the two dielectric layers, the magneto optical recording layer, for example, a TbFeCo layer, needs to be of an extremely thin thickness of 10 nm or less, thus leading to the playback signal level not more than one-half of the conventional signal level. The result is that it becomes difficult to raise the recording density such that the merit of the multiple recording layers cannot be exploited sufficiently. The same may be said of the phase transition type recording material.
In particular, with the phase transition recording layer, the recording state depends strongly on the cooling rate after laser light irradiation. That is, the phase transition type recording material is crystalline in the initial state and, on irradiation with the laser light, micro-sized areas thereof are raised in temperature to higher than its melting point and then cooled to form amorphous portions (pits). If the recording portion in its entirety is reduced in thickness and hence the metal reflective layer is reduced in thickness, an area of the metal. reflective layer heated to higher than the melting point is slowed down in the cooling rate thereof, because the metal reflective layer also performs the role of a heat sink. Thus the area is re-crystallized before cooling to ambient temperature. If the cooling rate is lowered further, the film material is deteriorated due to repetition of recording.
For the multi-layer optical disc, such a configuration may be envisaged in which the second recording layer is a recordable type recording layer and in which the laser light incident side first recording layer is a read-only recording layer. Such configuration is more likely to be implemented than the configuration of using the recordable type recording layer as the first recording layer because the read-only recording layer can be raised in transmittance by controlling its thickness or material type. However, although the playback output of an acceptable level can be realized from the second recording layer in this case, a laser power larger than that for reproduction needs to be supplied for recording on the second recording layer. For assuring transmittance sufficient for producing such laser power, it is insufficient to control the thickness or material of the first recording layer.